fix: bound streamed tool argument memory

[Ingwannu]

Summary

• cap shared SSE records at 4 MiB, including unterminated and multiline records
• cap retained tool arguments at 8 MiB per call and 32 MiB per turn across OpenAI Chat, the Responses bridge, and Chat outbound conversion
• fail oversized calls as typed upstream errors without emitting a completed tool item or clean Chat DONE marker
• document the limits and decision tradeoffs

Why

Compatible upstreams can omit SSE delimiters or stream unbounded tool argument fragments. Those buffers were protocol-required, but they had no byte ownership limit and could grow for the lifetime of a turn. Truncating and completing a call would produce invalid or unsafe executable input, so overflow now fails truthfully while preserving the existing sequential tool event contract.

Validation

• bun test on 15 related bridge, Chat, OpenAI Chat, and SSE suites: 206 pass
• bun run typecheck
• bun run privacy:scan

Related to #820. Focused alternative to the overlapping portions of #829.

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[Ingwannu]

CI follow-up: the first Windows run passed 6,514 tests and timed out only the unrelated existing image-normalization P2 case at 5.39 seconds against its 5-second test limit. Re-running the failed Windows job passed. Ubuntu, macOS, Windows, package-install, target-enforcement, label, and React checks are now green.